So drop in Galactus and Phoenix and let them run lose. Well both operate on limited energy reserves, so after expanding certain amount time both would need refueling. Then you have the case, that DC has its Heavy hitting cosmics. I seriously doubt they would not get involve, when outside Cosmic Forces threaten universal chaos.
I am happy if they manage to kill of Superboy Prime or Cyborg Superman (one needs to die, to shut his mouth, the other needs peace).